Non-tuberculous mycobacteria (NTM) are emerging pathogens that cause a spectrum of clinical -manifestations, ranging from asymptomatic colonization to severe, progressive lung destruction. The organisms are ubiquitous in the environment, and it is often difficult to determine the significance of any particular respiratory isolate. Therefore, clinicians are as likely to ignore an isolate that is truly causing NTM pulmonary disease, as they are to prescribe inappropriate and often toxic therapy to patients who do not truly have NTM pulmonary disease but are only transiently colonized. The goal of our proposal is to increase our understanding of NTM pulmonary disease from both the microbe and host perspective. Our proposal is driven by two hypotheses. First, we hypothesize that specific, genetically related isolates of Mycobactefium intracellulare will be found to be associated with the disease state. Our first Specific Aim is therefore a case-control study of patients whose respiratory secretions grow the most common subset of NTM, the M. avium complex (MAC). We will examine mycobacterial isolates from patients with pulmonary disease due to MAC (cases) as well as isolates from control patients. The control patients have been selected from a group of individuals with only a single respiratory culture growing MAC and who are highly unlikely to have NTM pulmonary disease. In the laboratory, we will separate them into phylogenetically related groups using variations in the sequence of the 16S-23S internal transcribed spacer. The association between specific genetically related subgroups and MAC pulmonary disease will then be explored. Our second hypothesis is that, at a minimum, radiographicatly identified bronchiectasis will be associated with NTM pulmonary disease. Furthermore, we believe we can identify a combination of host (demographic characteristics, comorbidities) and microbial (species and genetic subgroup) factors that will strengthen the ability to predict which patients who grow an NTM from a respiratory site will have NTM pulmonary disease. We address this hypothesis in Specific Aim #2 by combining retrospective and prospective data from patients who grow NTM from respiratory specimens submitted for mycobacterial culture. After assessing patients' baseline demographics, comorbidities, and mycobacterial isolate (as described for the first hypothesis), we will prospectively follow the patients, collecting additional respiratory samples and clinical data. The primary outcome will be a diagnosis of NTM pulmonary disease, as defined by the ATS. This research combines classic and molecular epidemiological methods to form a basis for future investigations of host immune susceptibility to NTM, mycobacterial pathogenesis, and improved treatment options. Our findings will also assist clinicians in predicting which patients with NTM isolated from the respiratory tract will have NTM pulmonary disease.